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      手动实现call，apply,bind
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        <h3 id="call-apply-bind是什么？"><a href="#call-apply-bind是什么？" class="headerlink" title="call apply bind是什么？"></a>call apply bind是什么？</h3><p>call，apply，bind是js给函数内部提供的api，可以通过调用他们改变函数内部的指向。</p>
<h4 id="如何使用"><a href="#如何使用" class="headerlink" title="如何使用"></a>如何使用</h4><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">getName</span>(<span class="params">age,home</span>)</span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="built_in">this</span>.name,age,home)</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">var</span> obj = &#123; <span class="attr">name</span>:<span class="string">&#x27;小朱&#x27;</span> &#125;</span><br></pre></td></tr></table></figure>
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<p><img src="https://ss1.bdstatic.com/70cFuXSh_Q1YnxGkpoWK1HF6hhy/it/u=450395766,216432551&fm=15&gp=0.jpg" alt="$nextTick"><br>我们想实现的是调用<code>getName</code> 打印出obj的name属性，我们可以用call，apply，bind分别来实现</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// call</span></span><br><span class="line">  getName.call(obj,<span class="number">10</span>,<span class="string">&#x27;北京&#x27;</span>) </span><br><span class="line"> <span class="comment">// apply</span></span><br><span class="line">  getName.apply(obj,[<span class="number">10</span>,<span class="string">&#x27;北京&#x27;</span>])</span><br><span class="line">  <span class="comment">// bind</span></span><br><span class="line"> <span class="keyword">const</span> newFun =  get.bind(obj,[<span class="number">10</span>,<span class="string">&#x27;北京&#x27;</span>]);</span><br><span class="line">   newFun();</span><br></pre></td></tr></table></figure>
<p>apply和call只是传参不一样，但是两个都是会在调用的时候同时执行调用的函数，但是bind则会绑定一个this的函数（改变this指向）。</p>
<h4 id="实现call-apply-bind"><a href="#实现call-apply-bind" class="headerlink" title="实现call, apply bind"></a>实现call, apply bind</h4><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br></pre></td><td class="code"><pre><span class="line">(<span class="function"><span class="keyword">function</span>(<span class="params">prototype</span>) </span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">getDefaultContext</span>(<span class="params">context</span>) </span>&#123;</span><br><span class="line">        <span class="comment">// 如果传入的是 null undefined context等于window</span></span><br><span class="line">        context = context || <span class="built_in">window</span>;</span><br><span class="line">        <span class="comment">// 如果基本值 &#x27;a&#x27; true 1</span></span><br><span class="line">        <span class="keyword">let</span> type = <span class="keyword">typeof</span> context;</span><br><span class="line">        <span class="keyword">if</span> ([<span class="string">&#x27;number&#x27;</span>, <span class="string">&#x27;string&#x27;</span>, <span class="string">&#x27;boolean&#x27;</span>].includes(type)) &#123;</span><br><span class="line">            context = <span class="keyword">new</span> context.constructor(context);</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span> context;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">call2</span>(<span class="params">context, ...args</span>) </span>&#123;</span><br><span class="line">        context = getDefaultContext(context);</span><br><span class="line">        <span class="keyword">let</span> symbol = <span class="built_in">Symbol</span>(<span class="string">&#x27;fn&#x27;</span>);</span><br><span class="line">        context[symbol] = <span class="built_in">this</span>;</span><br><span class="line">        context[symbol](...args);</span><br><span class="line">        <span class="keyword">delete</span> context[symbol]; </span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">apply2</span>(<span class="params">context, args</span>) </span>&#123;</span><br><span class="line">        context = getDefaultContext(context);</span><br><span class="line">        <span class="keyword">let</span> symbol = <span class="built_in">Symbol</span>(<span class="string">&#x27;fn&#x27;</span>);</span><br><span class="line">        context[symbol] = <span class="built_in">this</span>;</span><br><span class="line">        context[symbol](...args);</span><br><span class="line">        <span class="keyword">delete</span> context[symbol];</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">bind2</span>(<span class="params">context, ...outerArgs</span>) </span>&#123;</span><br><span class="line">        <span class="comment">// this = getName</span></span><br><span class="line">        <span class="keyword">return</span> <span class="function">(<span class="params">...args</span>) =&gt;</span> <span class="built_in">this</span>.call(context, ...outerArgs, ...args);</span><br><span class="line"></span><br><span class="line">    &#125;</span><br><span class="line">    prototype.call2 = call2;</span><br><span class="line">    prototype.apply2 = apply2;</span><br><span class="line">    prototype.binds = bind2;</span><br><span class="line">&#125;)(<span class="built_in">Function</span>.prototype);</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">getName</span>(<span class="params">age, home</span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="built_in">this</span>.name, age, home)</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">var</span> obj = &#123; <span class="attr">name</span>: <span class="string">&#x27;小朱&#x27;</span> &#125;</span><br><span class="line">getName.call2(obj, <span class="number">10</span>, <span class="string">&#x27;北京&#x27;</span>)</span><br><span class="line">getName.apply2(obj, [<span class="number">10</span>, <span class="string">&#x27;北京&#x27;</span>])</span><br><span class="line"><span class="keyword">let</span> bindGetName = getName.bind(obj, <span class="number">10</span>);</span><br><span class="line">bindGetName(<span class="string">&#x27;北京&#x27;</span>);</span><br></pre></td></tr></table></figure>
<p>这样 简易apply ，call，bind就这样实现了。</p>
<h5 id="结尾"><a href="#结尾" class="headerlink" title="结尾"></a>结尾</h5><p> 学习不会停止，希望以后多学习，丰富自己，多练习，摆脱各种学习上的困扰加油！！！</p>

      
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